Various forms of amplifier circuits have been used for years to provide an electrical output that is a function of a corresponding input parameter. Amplifier circuits increase the magnitude of the input by means of energy drawn from an external source, thus introducing gain. It is typically desirable for an amplifier to provide a linear response, meaning that the output signal of the amplifier closely resembles the input signal although the magnitude may be increased.
Amplifier circuits typically employ transistors and/or other active circuit components to introduce gain. Unfortunately, such circuit components are generally non-linear and, therefore, the output signal of an amplifier is not an exact replica of the input signal (at least over a relatively broad range of inputs). There are generally spurious components added to an amplified signal by the amplifier circuitry, such as in the form of harmonic generation or intermodulation distortion (IMD). These nonlinearities result in amplifier transfer functions that do not represent the ideal linear (straight line) amplifier transfer function.
One technique that has been used to address such nonlinearities has been to adopt a differential amplifier circuit configuration. In a differential amplifier circuit configuration, the amplifier circuit is constructed using a differential component configuration that multiplies the difference between two inputs (e.g., V+IN and V−IN) by a constant factor (the differential gain). The differential component configuration structure operates to cancel out second order nonlinearities associated with the individual components.
The foregoing differential amplifier circuit configuration, although generally providing acceptable linearity, is nevertheless unsuitable for use in many situations. In particular, the differential amplifier circuit configuration, by definition, requires two inputs (doubled-ended input). Moreover, to realize the advantages of the differential component configuration structure to provide canceling of nonlinearities associated with the individual components, the input signals must be balanced. However, many situations requiring signal amplification do not provide, nor readily accommodate, two input signals or double-ended input.
For example, television and broadband cable transmission systems almost universally utilize single signal configurations (single-ended). Such cable transmission systems often employ amplifiers, such as in transmission relays, set top converter boxes, signal splitters, etc. These situations are typically associated with single-ended input and single-ended output, thereby suggesting the use of single-ended amplifier circuit configurations. However, it is desirable for amplifiers used in cable transmission systems to exhibit good linearity, particularly in modern systems where a very broad band of frequencies are transmitted.
Single-ended amplifiers, however, are prone to undesirable distortion. For example, although it may be possible to drive down second-order distortion of a single-ended amplifier by substantially increasing the current, such an increase in current generally results in increased noise, decreased headroom, etc. Moreover, the use of such high current is not conducive to use in low power applications, which are common today.